A condenser microphone incorporates an impedance converter such as a field effect transistor (FET) or the like because the microphone unit thereof has a very high impedance. In the condenser microphone, a phantom power source is used, and microphone sound signals are sent via a balanced shielded cable of the phantom power source.
To connect the balanced shielded cable, a three-pin type output connector is provided on the microphone casing (microphone grip in a handheld microphone) side (for example, refer to Japanese Patent Application Publication No. H11-341583). The output connector is a connector specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment”, and the configuration thereof is described below with reference to FIGS. 3 to 6.
FIG. 3 is a sectional view showing a state in which the output connector is mounted in the microphone casing, and FIG. 4 is a front view of the output connector removed from the microphone casing. In FIG. 3, the output connector is shown in a cross section taken along the line A-A of FIG. 4. FIG. 5 is a plan view of the output connector.
According to these figures, the output connector 10 includes a disc-shaped connector base 11 formed of an electrical insulating material such as PBT (polybutadiene terephthalate) resin. In the connector base 11, three pins, that is, a No. 1 pin E for grounding, a No. 2 pin SH on the hot side for signal, and a No. 3 pin SC on the cold side for signal are penetratingly provided by force fit, for example.
Concerning the handheld microphone, as shown in FIG. 3, the output connector 10 is mounted in a connector housing cylinder 20 screwed to an end portion of the microphone grip, not shown, formed in a cylindrical shape. Usually, the microphone grip including the connector housing cylinder 20 is formed of a metallic material such as brass, and also functions as a casing for shielding the incorporated electrical parts.
In the connector base 11, a male screw 12 is provided to electrically connect the No. 1 pin E for grounding to the connector housing cylinder 20. The male screw 12 is housed in a screw housing hole 13 pierced in the radial direction in the connector base 11. Also, the connector base 11 is provided with an earth terminal plate 14 having internal threads 14a threadedly engaging with the male screw 12 in the screw housing hole 13.
As shown in the plan view of FIG. 5, the earth terminal plate 14 and the No. 1 pin E for grounding are electrically connected to each other via a connecting member 15. As shown in FIG. 3, the male screw 12 is brought into contact with the peripheral edge of a round hole 21, which is pierced in the connector housing cylinder 20, by turning the male screw 12 using a screwdriver, not shown, from the round hole 21.
Thereby, the No. 1 pin E for grounding and the connector housing cylinder 20 are electrically connected to each other via the male screw 12, the earth terminal plate 14, and the connecting member 15. In addition to this configuration, as shown in FIGS. 4 and 5, in some cases, a plate spring 16 contacting with the inner surface of the connector housing cylinder 20 is connected to the No. 1 pin E for grounding so that the No. 1 pin E for grounding and the connector housing cylinder 20 are electrically connected to each other by this plate spring 16.
Even if the No. 1 pin E for grounding is electrically connected to the connector housing cylinder 20 in this manner, when strong electromagnetic waves emitted from a cellular phone or the like are applied to the microphone or a microphone cable (balanced shielded cable) in the state in which the microphone cable pulled out of the phantom power source (not shown) side is connected to the output connector 10, in some cases, the electromagnetic waves intrude into the microphone through the output connector 10, being demodulated by the impedance converter, and are delivered from the microphone as noise having an audio frequency.
To solve this problem, the invention described in Japanese Patent No. 4273019 has disclosed a condenser microphone in which an electronic circuit for microphone unit is incorporated in the microphone casing, and the output connector is mounted to an end portion of the microphone casing. In this condenser microphone, the No. 1 pin for grounding is connected directly to the microphone casing and is connected to the ground of the electronic circuit via a high-frequency choke coil, and the ground is connected to the microphone casing via a lead wire, whereby high-frequency electromagnetic waves, which may cause the generation of noise, are prevented from intruding into the microphone casing through the output connector.
As another method for preventing electromagnetic waves from intruding into the microphone through the output connector 10, there is available a method in which, as shown in FIG. 5, capacitor devices C are connectingly provided between the No. 1 pin E for grounding and the No. 2 pin SH on the hot side and between the No. 1 pin E for grounding and the No. 3 pin SC on the cold side, and the No. 2 pin SH on the hot side and the No. 3 pin SC on the cold side are connected to the microphone casing including the connector housing cylinder 20 via an inductor device L for inhibiting high frequencies from intruding.
By a filter circuit consisting of the capacitor devices C and the inductor devices L, ordinary broadcast waves and electromagnetic waves such as HF, VHF, and UHF waves can be prevented from intruding almost without problems. However, a problem occurring when the condenser microphone is exposed to considerably strong electromagnetic waves emitted from a cellular phone is described below with reference to a schematic view of FIG. 6.
In FIG. 6, Z1 denotes an impedance of the filter circuit including the capacitor devices C and the inductor devices L, which are connected to the No. 2 pin SH on the hot side on the primary side of an output transformer T of the microphone, Z2 denotes an impedance of the filter circuit including the capacitor devices C and the inductor devices L, which are connected to the No. 3 pin SC on the cold side similarly on the primary side of an output transformer T of the microphone, Z3 and Z4 are impedances existing on the hot side and the cold side, respectively, of the balanced shielded cable for phantom power source. In order for noise to be canceled, design is made so that the impedances Z1 and Z2 are in balance, and the impedances Z3 and Z4 are in balance.
However, if the impedances Z1 and Z2 of each filter circuit are not in balance (imbalanced) in a high frequency region, when a high-frequency signal Ua is fed from a cellular phone or the like, an imbalanced signal Ub that is not canceled is generated between the hot side and the cold side, whereby a high-frequency current is caused to flow in the microphone casing including the connector housing cylinder 20, so that noise louder than ordinary is generated.
In the case where the output transformer T of the microphone is wired by using a relatively long lead wire, depending on the length and layout of the lead wire, the impedances Z1 and Z2 of each filter circuit become imbalanced even at frequencies of VHF band, so that noise may be generated.
Considering this issue, in the invention described in Japanese Patent Application Publication No. 2007-324804, as shown in FIG. 7, in forming a first filter circuit 102 for the No. 2 pin on the hot side and a second filter circuit 103 for the No. 3 pin on the cold side on a printed wiring board 100, both of the filter circuits 102 and 103 are substantially symmetrical (in FIG. 7, symmetrical in the up-and-down direction) with each other with respect to the X-X line passing through a middle-point line pattern 143, for example, connected to a middle-point tap of the output transformer.
That is, the first filter circuit 102 includes three land parts 121, 122 and 123, a common mode choke coil CL, two capacitor devices C1 and C2, and one inductor device L1. Similarly, the second filter circuit 103 includes three land parts 131, 132 and 133, a common mode choke coil CL, two capacitor devices C1 and C2, and one inductor device L1. These land parts and devices are arranged so as to be substantially symmetrical with each other, respectively, with respect to the X-X line.
Also, a No. 1 pin connection land 110 to which the No. 1 pin for grounding is connected and a No. 2 pin connection land 120 to which the No. 2 pin on the hot side is connected are arranged so as to be substantially symmetrical with each other with respect to the X-X line. Because of the space condition, the No. 3 pin on the cold side is connected to a No. 3 pin connection land on the back surface side of the printed wiring board 100, and the No. 3 pin connection land is pulled out to the top surface side of the printed wiring board 100 via a through hole wiring 130.
Also, according to the invention described in Japanese Patent No. 4273019, a high-frequency choke coil IL is connectingly provided between the No. 1 pin connection land 110 and a ground electrode 101. Although not shown in the figure, the No. 1 pin is connected directly to the microphone casing.
According the invention described in Japanese Patent Application Publication No. 2007-324804, the first filter circuit 102 for the No. 2 pin on the hot side and the second filter circuit 103 for the No. 3 pin on the cold side, including the printed circuit patterns mounting these filter circuits, are arranged so as to be substantially symmetrical with each other with respect to the middle point line pattern 143, for example, connected to the middle-point tap of the output transformer. Therefore, since the equilibrium of microphone output is maintained up to a high frequency region, even if the microphone is exposed to strong high-frequency electromagnetic waves emitted from a cellular phone or the like, a high-frequency current does not flow in the microphone casing, so that noise can be prevented from being generated by extraneous noise.
Unfortunately, according to invention described in Japanese Patent Application Publication No. 2007-324804, the high-frequency choke coil IL connectingly provided between the No. 1 pin for grounding and the ground electrode 101 is arranged on one side of the filter circuits 102 and 103, for example, so as to be imbalancedly on the first filter circuit 102 side in the example shown in FIG. 7, so that a problem described below is pointed out.
The high-frequency choke coil IL generates a high-frequency magnetic field on account of the feed of high-frequency signals, and is magnetically connected to the inductors (L1 and CL) on the signal side. This magnetic connection (mutual induction) depends on the distance between the parts.
Therefore, high mutual induction is generated between the high-frequency choke coil IL and the inductor close to the high-frequency choke coil IL (for example, the inductor device L1 in the first filter circuit 102), and in contrast, low mutual induction is generated between the high-frequency choke coil IL and the inductor far from the high-frequency choke coil IL (for example, the inductor device L1 in the second filter circuit 103).
For this reason, although the filter circuits 102 and 103 are arranged symmetrically, the symmetry of the high-frequency current collapses, whereby the high-frequency current is allowed to intrude into the microphone casing, so that noise is still generated in some cases.
Accordingly, an object of the present invention is to provide a condenser microphone in which a filter circuit for the No. 2 pin on the hot side and a filter circuit for the No. 3 pin on the cold side are arranged symmetrically on a printed wiring board incorporated in a microphone casing, and a high-frequency choke coil is provided to connect the No. 1 pin for grounding to a ground electrode, wherein a high-frequency magnetic field generated from the high-frequency choke coil is applied evenly to inductors included in the filter circuits.